1. Field of the Invention
This invention relates to surfaces patterned by imprint lithography and more particularly relates to methods and apparatus for creating topographically patterned substrates such as those used for patterned magnetic media.
2. Description of the Related Art
Traditionally, the storage capacity and the areal density of magnetic storage media have been limited by certain restraints such as material characteristics, manufacturing processes, metrological limitations, and the like. For example, conventional multigrain magnetic media is generally created by covering a flat substrate with a thin layer of magnetic alloy that forms clusters of atoms on the substrate surface known as grains. Each grain operates as a partially independent unit of magnetization subject to influence from other grains.
Data may be stored in tracks comprised of regions of alternating magnetic polarity. To increase data storage density, tracks may be made narrower, or the length of the regions of alternating polarity along the track may be reduced. Shrinking these dimensions generally requires that the size of the random grains in the media be reduced, so that sharp boundaries and sharp track edge boundaries can be defined by the magnetic write head. If grains are too large, the signal to noise ratio of the recording system suffers, and data errors are generated at an unacceptable rate. On the other hand, if the grains are too small, they may become unstable from thermally induced vibrations and spontaneously reverse their magnetic polarity (leading to loss of stored data) in a process known as the superparamagnetic effect. As a result of the superparamagnetic effect, the areal density of stable storage media has typically been restricted to around 150 Gbit/in2 for conventional multigrain magnetic recording media.
Recently, patterned media has been proposed as a growth path for increased density and capacity with good thermal stability. With patterned media, a highly ordered array of magnetic islands is formed on a surface. Due to their physical separation and reduced magnetic coupling to one another, the magnetic islands function as individual magnetic units, comprised either of single grains, or a collection of strongly-coupled grains within each island. Since these magnetic islands are larger than the individual grains in conventional media, their magnetization is thermally stable. High density is achieved by storing data in tracks just one island wide, rather than having to be wide enough to accommodate multiple (typically on the order of 10) random grains.
Patterned media can be formed by a variety of methods known to those skilled in the art. One method used to produce topographically patterned substrates is called imprint (or “nanoimprint”) lithography. While imprint lithography enables the formation of small features on a substrate, reduced surface flatness often occurs during island formation. The flatness of the media surface affects the head-media spacing, which directly affects the performance and storage capacity of the storage media. It is necessary to produce island features that are both sufficiently tall to provide magnetic isolation and have a sufficiently flat top to provide good head-media spacing control.
FIGS. 1A-1E depict a series of cross-sectional side views of a substrate 100 subjected to a prior art imprint lithography process. Imprint lithography processes such as thermal or UV-cure imprint lithography typically involve the formation of a topography in a resist layer 102 by pressing a topographically-patterned imprint master 104 against a resist-coated substrate 100 (FIG. 1A). In thermal imprinting, either the master 104, the sample substrate 100, or both, are heated to soften the resist 102 during the imprinting process. Upon cooling, the imprinted resist patterns or features 108 solidify and retain their imprinted shape after removal of the master 104 (FIG. 1B). In UV-cure imprinting, a transparent master 104 is pressed against a substrate 100 coated with a liquid photopolymer resist 102. After exposure to UV light through the master 104, the resist 102 polymerizes into a solid, leaving solidified topographic features 108 in the cured resist layer 102.
Due to limitations in the creation of the master 104 and in the nanoimprinting process itself, features 108 typically have some corner rounding. For patterns with very small (nm scale) features 108, such as the patterns used to create islands on patterned magnetic media, corner rounding may comprise a large fraction of the total feature area. The rounded shape in the resist features 108 can have detrimental effects on the subsequent pillar 112 formation in the substrate 100.
To form the desired features 108 in the substrate 100, the substrate 100 and the imprintable layer 102 may be exposed to ions 110 (FIG. 1C) during a directional etch process such as Reactive Ion Etching (RIE). The ions 110 combine with the material of the imprintable resist layer 102 and the substrate 100 and may form an uncharged gas which is pumped out of the etching chamber, leaving an etched pattern in the substrate 100. The imprintable resist layer 102 typically etches at a rate close to that of the substrate 100, resulting in the formation of rounded pillars 112 in the substrate 100 if the patterns are etched deeply into the substrate 100 (FIG. 1D). In addition, the imprintable resist layer 102 may subject to sidewall erosion during the etch process resulting in additional rounding of the pillars 112 of the substrate 100. Rounded pillars 112 in the substrate 100 cause several problems that can hinder the read/write process and prevent substantial improvements in storage media technology.
One problem arises when a magnetic layer 114 (FIG. 1E) is deposited over the surface of the substrate 100 to coat the tops of the pillars 112. Because the tops of the pillars 112 are rounded, the magnetic layer 114 is generally rounded as well. The resulting uneven surface negatively affects the head-media spacing, thereby reducing the effectiveness of the storage device.
Although the edge-rounding effect can be reduced by reducing the etch depth into the substrate 100, doing so may result in substrate pillars 112 with insufficient height to provide the needed magnetic isolation in the finished storage media. Another solution for reducing rounding would appear to be the use of higher aspect ratio (taller) resist patterns. Unfortunately, it is technically challenging to produce high aspect ratio features in the desired size range by nanoimprint lithography.
In one embodiment, the pillars 112 are approximately 20-30 nm in diameter. As a result of the etching process described above, rounding can create sloped edges that are approximately 5-10 nm in width, which significantly reduces the diameter of the desirable flat area at the top of each pillar 112. Although the flat area on a pillar 112 can always be increased by increasing the overall diameter of the pillar 112, larger pillars 112 result in low pillar density, which correspondingly reduces magnetic island density and data storage capacity for the finished magnetic storage media. Thus, a solution is needed to prevent the rounded edges of patterns in the resist 102 from being transferred to the etched pillar 112 of the substrate 100.
From the foregoing discussion, it should be apparent that a need exists for an apparatus and method that create a topographically patterned substrate that retains a flat surface without rounding of the features formed thereon. Beneficially, such a method and apparatus would reduce magnetic coupling between domains, increase the effective area sensitive to corresponding read/write mechanisms, and minimize the height requirements for movement mechanisms, thus improving the possible storage density, signal-to-noise ratio, and overall performance of a storage device.